lets say hypothetically you have too much angle between your left arm and the club shaft at address. WHen you swing centrifigul force straightens it out therefore making the club longer. still following me? since the club is therefor longer the after it strikes the ball it would be able to dig a 3 inch deep divot and that would be a big shock to you and the club, maybe even break it. So your body will subciosciously adjust. A couple possible adjustments you will make without realizing it? One would be to quit on the shot, another would be to lift up at impact.
if your practice swing and the real thing are not the same it is usually because an issue with your set up will not allow it to be, whether you conciously know it or not. an improper set up is the root FOR MOST ALL SWING FLAWS